It's possible that your float valve in the carb is or was leaking slightly when closed. Maybe crud in the seat. During normal driving you won't see any bad effect as you'll be using the gas faster than the leak. During the extended idle, the leak slowly overfills the bowl and one result can be flooding the engine to death, and making it too rich to easily restart. This depending on the carb, how well its sealed and the leakage rate. It also can be an intermittent problem that goes away as the crud washes through the seat or moves around in the valve.
A float valve that won't close with a bigger leak can often result in a shower of gas all over your hot manifolds and engine compartment, with obvious serious fire danger. I'd second the note to check out the fuel lines.
